server_settings: Use a reasonable default when User-Agent not specified.
server_settings: Use a reasonable default when User-Agent not specified.

Apparently, some Zulip clients don't send a User-Agent in their
requests to /server_settings, and this makes that not 500.

andersk/zulip,tommyip/zulip,brainwane/zulip,tommyip/zulip,zulip/zulip,zulip/zulip,dhcrzf/zulip,andersk/zulip,zulip/zulip,kou/zulip,synicalsyntax/zulip,kou/zulip,brainwane/zulip,dhcrzf/zulip,hackerkid/zulip,brainwane/zulip,andersk/zulip,tommyip/zulip,timabbott/zulip,dhcrzf/zulip,eeshangarg/zulip,synicalsyntax/zulip,timabbott/zulip,andersk/zulip,tommyip/zulip,kou/zulip,shubhamdhama/zulip,shubhamdhama/zulip,kou/zulip,rht/zulip,brainwane/zulip,andersk/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,showell/zulip,punchagan/zulip,zulip/zulip,synicalsyntax/zulip,eeshangarg/zulip,punchagan/zulip,punchagan/zulip,dhcrzf/zulip,shubhamdhama/zulip,showell/zulip,tommyip/zulip,rishig/zulip,hackerkid/zulip,dhcrzf/zulip,timabbott/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,dhcrzf/zulip,tommyip/zulip,brainwane/zulip,timabbott/zulip,synicalsyntax/zulip,eeshangarg/zulip,hackerkid/zulip,rishig/zulip,timabbott/zulip,rht/zulip,rishig/zulip,rht/zulip,kou/zulip,rishig/zulip,eeshangarg/zulip,hackerkid/zulip,rht/zulip,tommyip/zulip,zulip/zulip,shubhamdhama/zulip,rht/zulip,punchagan/zulip,punchagan/zulip,brainwane/zulip,kou/zulip,showell/zulip,rishig/zulip,hackerkid/zulip,shubhamdhama/zulip,synicalsyntax/zulip,zulip/zulip,shubhamdhama/zulip,kou/zulip,timabbott/zulip,showell/zulip,rishig/zulip,showell/zulip,shubhamdhama/zulip,timabbott/zulip,hackerkid/zulip,rishig/zulip,showell/zulip,rht/zulip,eeshangarg/zulip,dhcrzf/zulip,andersk/zulip,punchagan/zulip,brainwane/zulip,synicalsyntax/zulip,showell/zulip